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Zijian Li
(Advisor: Prof. Christopher Reinhard) will defend a doctoral thesis entitled,
Thallium isotope investigation of paleo ocean redox and carbon dioxide removal via enhanced rock
weathering
On
Friday, July 15 at 2:00 p.m.
ES&T Classroom L1125
https://gatech.zoom.us/j/2527896823
Abstract
The oxygenation of Earth’s surface fundamentally reshaped global biogeochemical cycles, and surface oxygen levels
have played a critical role in the origin and diversification of metazoans. Stable thallium (Tl) isotope systematics
are mechanistically linked to the burial of manganese (Mn) oxides, making the system an effective redox proxy to
track free oxygen (O2) levels in the ocean. The Mesoarchean and mid-Proterozoic are two critical periods of time in
geologic history for the evolution of microbial and complex life. In the first half of this thesis, I analyze Tl
isotopic compositions of mid-Proterozoic black shales and Mesoarchean siliciclastic sediments and perform stochastic
modeling of marine Tl isotope mass balance to extract paleo-redox conditions that have been poorly constrained. The
χ205Tl composition of the upper Velkerri Formation is indistinguishable from the crustal value, which
implies the global burial of Mn oxides was limited at 1.36 Ga and contemporaneous deep ocean was pervasively
anoxic. The strong positive χ205Tl values of the 2.95 Ga Sinqeni Formation are not likely to
represent global seawater records, but rather preserve a primary signal of localized Mn oxides
burial in Mesoarchean marine sediments. Given the free O2 is required to stabilize Mn oxides
against reductive dissolution during settling, our results provide strong evidence for the early
emergence of oxygenic photosynthesis at least 3 billion years ago.
There is increasing consensus that immediate and deep reductions in greenhouse gas (GHG) emissions are
necessary in the coming decades to limit global warming to 1.5°C target (the Paris Agreement).
Carbon dioxide removal (CDR) from Earth’s atmosphere is likely to play a significant role in
achieving the climate mitigation goals. The second half of this thesis focuses on enhanced rock
weathering (ERW), a negative-emission CDR strategy that spreads milled calcium- and magnesium-rich
silicates (or alkaline materials) on croplands or in the ocean to artificially speed up the
weathering process and associated atmospheric CO2 removal. A hierarchy of models is adopted to
evaluate the CDR potential and environmental impacts of ocean-based ERW using natural and synthetic
mineral feedstocks. Compared to olivine and basalt, application of alkaline metal oxides (CaO and
MgO) leads to higher CDR efficiency with reduced environmental impacts, but deployment at scale
faces challenges of substrate limitation and process CO2 emissions. I then
perform an analysis of the energetic and economic demands of rock grinding, the most energydemanding
and cost-intensive step in the ERW life cycle, and conduct state-level assessment of
carbon footprints, costs, and energy requirements associated with grinding for the U.S. The results
of geospatial analysis highlight the regional differences in deploying grinding and indicate that the
operation of grinding in the U.S. is generally cost-effective and energy-efficient based on the
nation’s average electricity mix.
Committee
• Prof. Christopher Reinhard – School of Earth and Atmospheric Sciences (advisor)
• Prof. Yuanzhi Tang – School of Earth and Atmospheric Sciences
• Prof. Ellery Ingall – School of Earth and Atmospheric Sciences
• Prof. Jennifer Glass – School of Earth and Atmospheric Sciences
• Prof. Jeremy Owens – Department of Earth, Ocean, and Atmospheric Science, Florida
State University
• Prof. Shuang Zhang – Department of Oceanography, Texas A&M University
